There has been a rapid development of the semiconductor industry recently and an acceleration of the integration rate of realizing miniaturization and high performance of a unit device. Thus, the operating voltage of electronic equipments or the like has a tendency of being gradually reduced. Conversely, when a surge voltage is applied, thermal energy, high enough to burn electronic parts, is generated, thus rapidly reducing the energy carrying capacity of a semiconductor. Therefore, the capability to cope with surges has dramatically been reduced.
As equipments including semiconductor devices are susceptible to excessive voltage, such devices may be destroyed or degraded when excessive voltage is supplied for even a relatively short duration of several microseconds (μs), thus reducing the lifespan of equipments or deteriorating the functionality of the equipments. For these reasons, it is necessary to develop a varistor that can be operated at micro-voltage.
A varistor is defined as a semiconductor device having a highly nonlinear volt-ampere characteristic. The electrical characteristics of the varistor are similar to the function of a Zener diode, which has constant voltage characteristics. However, the varistor is a composite ceramic device that has larger current and energy capacity.
The nonlinear characteristic of the varistor has very large electric resistances at low voltages. The resistance has a grain boundary phenomenon showing the nonlinear characteristic. That is, when the voltage exceeds a predetermined threshold voltage depending on the microstructure and size of the device, the electric resistance is abruptly reduced.
The nonlinear resistance characteristic is controlled by a process that occurs at the grain boundary. The nonlinear resistance characteristic is similar to a breakdown observed at a back-to-back Zener diode, but has larger energy absorbing capability.
Varistors have been used to stabilize voltage, quench sparks of contact points, or absorb surges in electronic circuits. Further, the varistors may be employed in an arrester for protecting an electrical system from lightning.
As shown in FIG. 1, the method of manufacturing a conventional disc varistor is as follows. That is, an electrode 2 is applied to a surface of a ceramic body 1, while an electrode 3 is applied to the opposite surface of the ceramic body 1. Lead wires 4 and 5 to be connected to a circuit (e.g. printed circuit board) are soldered respectively to the electrodes 2 and 3. Afterwards, a coating process with an insulating material comprising epoxy 6 is carried out. In this regard, the varistor having the electrodes 2 and 3 on opposite surfaces thereof is manufactured.